1. Field of the Invention
The subject invention relates to an electrodialysis method, and more particularly, to an electrodialysis method for the treatment of a silicate-containing potassium hydroxide etching waste solution.
2. Descriptions of the Related Art
Owing to such advantages as high flexibility in operation, small space occupation of equipment, high purity of resulting products and good adaptability to conductivity of materials to be processed, the electrodialysis (ED) process is known as a waste solution process that is commonly used, for example, for radioactive waste solution treatment, recovery of valuable metals from waste electroplating solutions, etc. Also, the electrodialysis process may also be used in such processes as production of table salts or pre-treatment of boiler water. The principle of the electrodialysis process is that the ion exchange membranes having distinct properties are used to selectively separate ions in the water, and the migration of the ions in the water is driven by positive and negative direct currents (DC) to attract ions. In other words, this process utilizes the characteristic that the cations can only penetrate through cation exchange membranes while the anions can only penetrate through anion exchange membranes. Under the action of the applied DC electric field, the anions in the water migrate towards the anode while the cations in the water migrate towards the cathode, thereby accomplishing the purpose of recovering valuable substances from the waste solution.
The etching waste solutions of general semiconductor industries contain in addition to strong alkaline etching solutions (e.g., potassium hydroxide), also silicon dioxide etched away from semiconductor substrates and metal oxides (e.g., potassium oxide) formed during the reactions. The silicon dioxide and metal oxides often form in the waste etching solutions colloidal silicates, which are generally known as water glass and may be normally represented by a general formula MxOy.nSiOx (where M represents a metal such as Na or K, coefficients x and y vary with species of the metal, and n also varies with species of the metal and is within a specific range). Therefore, if the etching waste solutions are directly introduced into a single-membrane electrodialysis system for recovery, the silicates contained in the etching waste solutions tend to cause clogging to the ion exchange membrane of the electrodialysis system, resulting in deactivation or even loss of the ion exchange capability thereof. Moreover, the anode is also liable to be surrounded and absorbed by the silicates, and consequently fails in efficacy. Hence, no electrodialysis method has been provided up to now that is capable of effectively treating silicate-containing waste solutions for separation and recovery of useful substances therefrom.
Etching solutions currently used in semiconductor industries are mostly potassium hydroxide or sodium hydroxide solution. If such etching waste solutions are subjected to the waste treatment or directly used to produce products of low economic values, poor economic benefits would result.
In view of this, the subject invention provides an electrodialysis method capable of treating a potassium silicate-containing waste solution and solving the problem of clogging the ion exchange membranes during electrodialysis. This method can not only recover potassium ions from the waste solution effectively and economically, but also recover potassium silicates with a low content of potassium for use in other industrial applications (e.g., for use in producing water glass after being concentrated), thereby achieving the goal of waste reduction and resource recycling effectively.